Thermoplastic polyurethane (TPU) is a versatile elastomer that shows a good performance regarding resistance to oils and grease, tear and abrasion resistance, low temperature flexibility, resilience and tensile strength, but this material is also noted for having “poor to fair” hydrolysis resistance and a relatively high cost. TPU is a block copolymer that owes its elastic properties to the phase separation of so-called “hard blocks” and “soft blocks”. Hard blocks are rigid structures physically cross-linked that give the polymer its firmness; soft blocks are stretchable chains that give the polymer its elasticity. It is worth noting that the presence of polar and non-polar counterbalanced microdomains in the TPU structure is the cause of its good chemical resistance, particularly oil and grease resistance.
TPU is commonly used in footwear, automotive and electronics products. Furthermore, TPU is a component of hoses, belts, tubes, products of the industrial machinery and the like, but it has the drawbacks of poor hydrolytic and weather resistance, and hence its uses are limited.
TPU is the reaction product of a diisocyanate, a chain extender (a short chain diol) and a polyol, wherein urethane groups are formed along the polymer chain. TPU can be produced in several ways, but the most common process is a reactive extrusion, wherein a polyol containing poly-hydroxyl compounds, chain extenders, additives, and isocyanate compounds are fed into an extruder in a precise ratio aiming to achieve the properties needed for the final application. However, reactive extrusion method is not flexible enough to obtain all the desirable properties.
On the other hand, styrene block copolymers (SBCs) are widely used as elastomers in industry due to their excellent mechanical properties, elasticity and hydrolytic resistance. Furthermore, SBCs exhibit excellent weather resistance when hydrogenated. SBCs are also block copolymers that owe their elastic properties to the phase separation of “hard blocks” and “soft blocks”, which gives the polymer its firmness and its elasticity respectively. Nevertheless, SBC uses are limited due to their poor resistance to oil and abrasion, among other drawbacks.
Polar resins like polyurethanes are incompatible, and hence hardly blended or mixed with polyolefins such as polyethylene, polypropylene, or with diene-based elastomers such as SBCs (“Polymer Blends” by D. R. Paul and S. Newman, Volume 1, 2, Academic Press, Inc., 1978//Thermoplastic Elastomers. RP Quirk). Despite of this, the co-processing of TPUs and SBCs has been attempted by means of mechanical blending or by compounding with an extruder (co-extrusion) in order to achieve a more intimate mixing.
The incompatibility of both polymers results in non-homogeneous blends which tend to delaminate, and often feature poor mechanical properties. Another disadvantage is that the compounding process, since it is done at relatively high temperatures, has a detrimental effect on the physical properties of the modified TPU produced that way, since the polymers will undergo a thermal degradation during processing. A further disadvantage is that the production is long and costly.
Aiming to prevent separation of the SBC from the TPU and obtaining an homogeneous mixture with properties that combine the ones of SBC and the ones of TPU, compatibilizing agents have been used.
For instance, WO99/29777 describes the use of a copolymer obtained as the reaction product of a maleic anhydride grafted polypropylene and a polyamide. This compatibilizing agent is used in blends of non-polar EPDM and thermoplastic polyurethane or polyvinylidene fluoride. In WO 2011/077234, styrene and ethylene/butylenes grafted with maleic anhydride onto the rubber mid-block are used to improve compatibility of these polymers with TPU. In both cases, the presence of maleic groups along the polymer chain leads to crosslinking reactions that are difficult to control, yielding polymer mixtures with high viscosity and low processability.
U.S. Pat. No. 5,925,724 and EP0994919B1 teach the use of optionally hydrogenated polybutadiene diols that are added to a TPU formulation, thus reacting with isocyanate groups and forming a TPU/polybutadiene hybrid polymer. In U.S. Pat. No. 5,925,724, the TPU composition is prepared by a prepolymer method. In EP0994919B1, the resulting polymer has improved compatibility with polyolefin compounds. In order to ensure a good mixture with the polyurethane components, short chain polybutadiene diols are required. This feature decreases mechanical properties, and leads to a poor phase separation of the final product and hence, limited compatibilization properties.
Some other products include styrene based block copolymers. For example, EP0611806 and U.S. Pat. No. 7,138,175 use SBCs functionalized with OH groups and reacted with TPU. In the first document, SBCs containing isoprene and OH groups are blended together with TPU at 200° C. In order to control the reaction rate, a catalyst deactivator like distearyl phosphate is required. This substance is used as antifoaming or to prevent the extensive occurrence of ester interchange during blending. The resulting process is costly and it does not solve the problems of polymer degradation during blending. In U.S. Pat. No. 7,138,175, SBCs containing OH groups are reacted with the polyol and the isocyanate compounds in the feed zone of an extruder, and a functionalized styrene copolymer is added in the compression zone of the said extruder. According to this document, hydroxyl functional groups on the SBC are required in order to react with the polyurethane product and improve compatibilization of the TPU and block copolymers. The main disadvantage of these methods is the limited reactivity of hydroxyl functional groups with isocianate groups as mentioned in Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 40, 2310-2328 (2002)), which leads to low reaction efficiency. Another disadvantage is that the compounding process, since it is done at relatively high temperatures, has a detrimental effect on the physical properties of the styrene-based block copolymers, since the polymers will undergo a thermal degradation. A further disadvantage is that removing undesirable reaction products is long and costly.
For the several drawbacks existing in the processes described above, there is the need of developing compatibilizers of TPU and SBC elastomers, aiming to achieve improved properties in the final product blend.
Other kind of products that can be improved with the compatibilization between thermoplastic polyurethanes and styrene block copolymers are polyurethane (PU) foams. These materials are widely used in upholstery, bedding, cushioning, insulation panels, footwear and many other applications. The basic chemistry of polyurethane foams and thermoplastic polyurethanes is alike, consisting in the formation of urethane linkages from the reaction of polyols and isocyanates. It is provided that the introduction of SBCs to the PU foam structure will confer better properties, especially mechanical, to the resulting material.
In this sense, US2013/0316164 describes a PU foam prepared by introducing a plastiziced triblock copolymer gel into a mixture of polyurethane foam forming components including a polyol and an isocyanate. The plasticized triblock copolymer gel is previously prepared mixing a styrene ethylene/butylene styrene triblock copolymer (SEBS) with oil. The PU foam obtained has improved thermal conductivity, improved heat capacity, and higher support factors. Nevertheless, the synthesis of the foam does not occur due to the poor compatibility between the elastomer and the polyurethane foam forming components; thus, collapsing of the foam or non-homogeneous blends with non-homogenous properties are often observed.
Consequently, the addition of a suitable compatibilizer between SBCs and polyurethanes to the PU foam is desirable to obtain high performance materials.